Well, if the gecko is in a 40 gal, then get an adult. A baby would freak out in that size of an enclosure. I'd start with one at first, then slowly get more. You can connect with an adult, I have, but it is easier to connect with a younger one. If you use sand, do something like this:
http://easttnherps.com/yak/index.php?topic=5.0And don't use repti-sand. Use washed and sifted play sand.
Unless you can pay for the higher end Enigmas, don't buy one. The lower end ones have major problems. The spin around and such, which can make it difficult to get them to eat.